In the broadest sense, this refers to a type of investment company that holds a fixed portfolio of securities for a specified period of time. More specifically, it is also a structure used by some older ETFs that prevents them from making distributions until the end of each quarter, from holding securities not in the indexes they track, and from lending out securities. The inability to reinvest dividends daily, as newer ETFs and conventional mutual funds may do, can lead to tracking error, in which the ETF's performance diverges from its index. No new ETF has used this structure since 2002.